Many small electric motors, particularly those used for powering automotive appliances, have housings formed from deep drawn or rolled and seam-sealed steel. The housings are cup-shaped and also function as flux rings and bearing retainers. The permanent magnets are inserted into the steel housings and secured in place against flux ring portions of the housings by a variety of fastening techniques including adhesives, spring clips, screws, and rivets.
Such fastening techniques are laborious and costly and lower the reliability of motor assemblies. The permanent magnets mounted in the housings are brittle and subject to cracking or breaking. Corners of the permanent magnets chip quite easily, particularly as a result of the handling required to mount and secure the magnets in the housings. Broken fragments of the magnets can become lodged between moving parts and compromise motor performance. Motor rejection rates can be quite high for this reason alone.
Composite motor housings combining molded resin housings and steel flux rings are a viable alternative to drawn or rolled steel housings. One example found in U.S. Pat. No. 5,584,114 to McManus, a co-inventor of this application, provides for overmolding a flux ring and an end bearing in place within a resin housing. Protrusions molded through openings in the flux ring provide mounting features for press-fitting magnets against the flux ring.
While this press-fitting technique is a more reliable way of mounting the magnets, other issues have delayed a wider commercial acceptance of composite motor housings. Drawn or rolled steel housings can be made quite cheaply and have the inherent advantage of functioning as flux rings.
U.S. Pat. No. 3,789,250 to Macoit et al. discloses a resin housing around which two flux members are separately attached. However, the two flux members do not support a closed flux path surrounding the magnets, which is important to support motor performance. Structural portions of the resin housing separate permanent magnets from a rotor path, which can further detract from performance.
Other housings are made with steel flux rings and resin or steel end caps. The steel flux rings can be shaped from sheet metal, and the end caps can be machined, cast, or molded in a simple form to support end bearings. Permanent magnets are still mounted similar to those in drawn steel cans and are subject to the same consequences.